Technical Field
The present invention relates to semiconductor devices and, more particularly, to reducing dark current in germanium photodiodes.
Description of the Related Art
A photodiode is a semiconductor device that converts incoming light into an electrical current. As the photodiode absorbs incoming photons, the energy is converted to electrical energy that can be used for signaling or power generation. Semiconductor-based photodiodes are used in a variety of applications, including, for example, optical switching in photonic circuits, receiving signals in fiber optic communication systems, etc.
The material used to fabricate the photodiode dictates its properties and, in particular, the region of the electromagnetic spectrum to which the photodiodes sensitive. Photodiodes can, for example, be fabricated by depositing germanium diodes on a silicon substrate, providing sensitivity to longer wavelengths than would be accessible to silicon photodiodes. However, the resulting germanium-on-silicon diodes exhibit a high “dark current,” which is the current generated by the device when the device is not illuminated. In some cases, the dark current can be in excess of hundreds of nanoamps. Dark current may result from background radiation and the saturation current of the semiconductor junction forming the diode. One possible cause of the high dark current is a high defect density that results from the large lattice constant difference between the silicon and germanium layers. The shot noise from the high dark current reduces the signal-to-noise ratio of the photodetector, making it more difficult to capture faint signals. The variable nature of the dark current over temperature and between devices prevents accurate monitoring of the received optical signal level.